Sheldon Cooper of The Big Bang Theory was once asked by his neighbor Penny to explain physics to her.
He asked her to imagine herself in Ancient Greece, sitting under an evening sky, looking at the heavens above.
When ancient Greek astronomers did just that, they noticed some stars wandered against the firmament. They called these wanderers "planets" and they have captured our imagination ever since.
It was a lyrical telling of the beginning of physics and well in tune with the science that is portrayed in The Big Bang. It wasn't wrong, but it was very Sheldon.
There were five planets known in ancient times. Astronomers in all ancient civilizations - not just Ancient Greece - noticed their peculiar behavior. Most societies had stories of some sort to describe the behavior of these strange stars.
As civilization grew and as astronomers studied the night time skies in more detail, the paths that the wanderers grew in clarity. Observations and calculations by the likes of Copernicus, Brahe, and Kepler led to the development of a heliocentric view of the solar system. The Sun and not Earth was the centre of all.
Then along came lenses of sufficient quality and strength that Galileo could build a telescope and bring the Universe into sharper focus.
His observations of Jupiter included details of the surface, such as the Giant Red Spot, but also allowed Galileo to detect four of Jupiter's moons. It confirmed to astronomers that the Solar System is heliocentric.
After all, if bodies could orbit Jupiter, then surely the planets could orbit the Sun. And the Ptolemaic model couldn't even come close to explaining Galileo's observations.
But what kept everything going around? What kept the planets in their orbits? What allowed the heavens to be?
The answer is simple - gravity. However, it took the likes of Isaac Newton and the invention of calculus to really work things out. Gravity as a force of nature led physics to our modern understanding of the cosmos. This is where Sheldon was going with his story of an ancient Greek night.
Astronomy has changed over the past three hundred years and has changed by leaps and bounds in the past 20 or so.
With the discovery of the telescope, astronomers were able to examine all of the stars in the sky. Most are just points of light but the planets still stood out. More moons were discovered around Jupiter, Saturn, and Mars.
Then, more planets were discovered. In 1781, the blue sphere of Uranus was detected by Herschel. Nepture was first observed in 1846 and poor little Pluto in 1930.
Of course, Pluto lost its planetary status recently.
It wasn't only planets, though, that astronomers were finding throughout the solar system. Asteroids were discovered in a belt between Jupiter and Mars. Comets were observed in the outer reaches of the solar system long before they were visible to the naked eye.
And moons. Lots and lots of moons. At last count, there are 166 moons in the Solar System if you don't count the five around Pluto. As of last week, I think that number is now 167.
A new moon was discovered in old images from the Hubble Space Telescope. (The Hubble Space Telescope is arguably the single best investment that has ever come out of the space program.)
The new moon appears as little more than a constant blip in the pixels that define the space surrounding Neptune. However, the fact that it is there every time the telescope looks and has orbital characteristics consistent with a moon led to the announcement.
Yes, the solar system is getting to be a crowded place. Better instrumentation has led to a better understanding of our celestial neighbourhood.
Our explorations have stretched well past the local region of space, though. Modern telescopes have allowed astronomers to observe minute shifts in stellar position indicative of planetary bodies. Since 1988, exoplanets or planets orbiting other stars have been an addition to the nighttime sky.
As of July 12, 2013, a total of 910 exoplanets have been discovered, including 140 found in multiple planetary systems. Furthermore, the Kepler space telescope has identified an additional 262 likely targets for further exploration.
Perhaps most amazing is the announcement this week by NASA that astronomers using the Hubble Space Telescope have found another blue planet.
HD 189733b is 63 light years from Earth and yet as it passes across the face of its star, scattering of blue light confirms the colour of the planet. Indeed, it would look a lot like Earth as seen from space. Unfortunately, that is where the comparison ends as HD 189733b is a gas giant too close to its star for life to exist.
Who could imagine all of this from sitting under the stars and watching the night sky in Ancient Greece?
